Sound reproducing device of the electrodynamic or movingcoil type



March 13, 1934. LWMURKHAMETAL 1,951,311

SOUND REPRODUCING DEVICE OF THE ELECTRODYNAMIC OR MOVING COIL TYPE Filed Jan. 21, 1933 2 Sheets-Sheet l March 13, 1934. L w MURKHAM ET AL 1,951,311

SOUND REPRODUCING DEVICE OF THE ELECTRODYNAMIC OR MOVING COIL TYPE Filed Jan. 21, 1953 2 Sheets-Sheet 2 Patented Mar. 13, 1934 UNITED STATES PATENT OFFICE THE ELECTRODYNAMIC COIL TYPE Leonard Walter Murkham Bennett, London,

OR. M.O V IN G- and Douglas Temple England Application, January 21, 1933,. Serial No. 652,854 In Great Britain February 20, 1932 8 Claims.

This invention relates to instruments of the eIectro-d'ynamic'. or moving-coil type for use in theareproduction of sound, that is to say for use for producing. sound from electric current. of

& audio-frequency or for producing audio-frequency' electric currents from: sound waves.

There are: in. common use two main classes of loud. speaking instruments of the type in question for producing sound waves from audio-frem; quency current, namely the horn type, in which a small vibratory diaphragm serves to energize an; air column contained within a horn, and the large. diaphragm. type, in which a comparatively large: diaphragm, usually of conical formation,

acts in. conjunction with a bafiie-board" to impart vibratory movement to the surrounding air. With a view to economy in the magnet employed inamovingf-coil loud speaker, it is desirable that the annular gapthereof should beof the smallest 9; possible radial dimensions, and little or no difficulty' is encounteredin this direction in the case of the. small diaphragms which work in conjunction with a horn. Considerably' greater difliculty, however, is encountered in connection with large diaphragm type loud speakers, because, for the same loudnesslevel of reproduced sound, without. a horn, the amplitude of the vibrations even of a large diaphragm, must be greater than where a horn is employed; Small loud speakers intended fordomestic use are almost invariably of the large diaphragm type: in view of the large amount of room taken by an. adequate horn, and it is in such: loud speakers that it is particularly important for reasons of cost to employ magnets 5; having: annular gaps of small radial width. In. such. loud speakers, many different arrangements have been proposed for maintaining the movingcoil properly central and in proper alignment thean-nular gap-of its associated magnet. Var

ions electro-dyna-mic or moving-coil instruments for usezin: the production of audio-frequency electric: current from sound waves are also known already.

The. present invention has for anobject to provide improvements in: electro-dynamic or moving-coil microphone or sound reproducing instruments.

For use in the reproduction, or recording, of sound waves, it has already been proposed to- 50, provide in: combination, and, inter alia, for application to the known moving coil type of. instrument, a vibratory flat or disc diaphragm of thin sheet metal and a hollow truncated come the smaller end of which is secured to' the flat 55. vibratory diaphragm along a circular or annular region of the latter that is inset from the outer periphery of the free vibratory portion thereof;

According to the present invention, however, a composite diaphragm, for use in an instrument of the kind referred to comprises two elements, to the one being formed. from sheet metal as a flanged dome and having the former of the moving coil attached thereto as by adhesive so that. the moving coil extends coaxially thereof at the junction of the dome with the flange; and the 85 other being of conical formation and coaxially attached as by adhesive at its smaller periphery to the convex. surface of; the dome.

The inventionv is illustrated by the accompanying drawings, of which Figure I shows in half sectional side elevation, aloud speaker according to one; form of the in- Vention, part of the magnet thereof being broken away; and

Figure 2 is a fragmentary section view of an 76 alternative arrangement of loud speaker; whilst Figure 3 shows a microphone in. part sectional side elevation.

As shown in Figure I, a loud speaker comprises, apart from the vibratory diaphragm, a magnet, indicated at 11, having an annular gap, indicated at 12, of small radial width, anda centre. pole piece 13', and a conical support 14 for the vibratory diaphragm. The diaphragm comprises a conical element 15, similar to that of the large diaphragm loud speaker, formed from: paper in the wellknown manner, but without a flange at its larger periphery, to which is secured, as by adhesive, a surround 16 of flexible material, such for example, as thin leather, and a smaller, comparatively rigid element, like the diaphragm of a horn type loud speaker, formed by pressing from thin sheet metal or metallic alloy, with a central domed portion 17 and with a wide flange 18 formed with concentric corrugations 19; the two elements of the D5 diaphragm as a whole being secured cor-axially together, as by adhesive, as indicated at 20, at

moving coil, indicated at 2-1, is wound on a thin paper former, 22, the upper margin of which is )0 serrated, and secured by adhesive to the smaller diaphragm element, as at 23, at the junction. of the domed portion 17, with the flange 18 thereof. The two diaphragm elements 15 and 17, 18 and the moving coil 21, are disposed co-axially of one: another. The conical support 14 is formed. at its" smaller end with an apertured inturned flange 24 and with a further angle section flange 25 at its larger periphery, and also with the usual openings, indicated at 26; whilst an annular clamping member 27 of channel section and lined with felt 28, is secured by screws, one of which is indicated at 29, to the flange of the support 14.

In the production of a loud speaker such as that indicated in Figure 1, the smaller diaphragm element 17, 18 is first formed as above mentioned, by pressing from thin sheet metal, then the moving coil 21, already wound on its former 22, is secured to the smaller diaphragm element 17, 18 in the manner explained; an apertured washer is placed on the magnet 11 so that apertures therein register with corresponding threaded bores, one of which is indicated at 31, in the magnet 11; then the small diaphragm element and moving coil assembly is placed in position, with the moving coil 21 located in the gap 12 of the magnet 11, with apertures in the outer margin of the flange 18 in register with those of the washer 30 and of the magnet 11, two further apertured washers being placed one below and one above the flange 18; then, the support 14 is placed in position with the apertures in its smaller flange 24 in register with those just mentioned, and all the apertures, with the exception of the bores 31 of the magnet 11, being over size. All these parts are secured firmly together by screws, of which one is indicated at 32, passed through the various apertures. The over size apertures in the flange 24 and in the surround 18 permit proper centering of the moving coil 21 in the gap 12 before the screws 32 are finally tightened, and once these are tightened, there is no possibility of the coil 21 or smaller diaphragm element 17, 18 becoming displaced laterally. Next,

the clamping member 27 and screws 29 being removed from position, the conical diaphragm element 15 is placed in position truly co-axial with the smaller diaphragm element 17, 18; the clamping ring 27 is secured in position by the screws 29, two washers being disposed, the one above, and the other below, the surround 16 of said element 15, and the inner end of the diaphragm element 15, which abuts against the domed portion 17 of the smaller diaphragm element 17, 18 is secured thereto by adhesive, as indicated at 20.

Leads (not shown) from the moving coil 21 may be brought out in any known or convenient manner to suitable terminals (not shown).

In the form shown in Figure 2, a similar magnet 41, support 42 and conical diaphragm 43, are employed. The smaller diaphragm element, however, in this case is formed by pressing with a domed portion 44 and with a flat flange 45 which is not formed with corrugations. In this case also, a moving coil 46 is attached to the small diaphragm 44, 45 in the same way as in the case of Figure 1. The smaller diaphragm 44, 45 is further provided with a separate surround 47 of comparatively flexible thin sheet material, such as celluloid, which is secured to the flange 45 of the smaller diaphragm element 44, 45 by adhesive. The outer margin of this surround 47 is mounted by means of screws, one of which is indicated at 48, passed through oversize holes in an inturned flange 49 of the support 42 through oversize holes in packing washers and in said surround 47 into threaded apertures, one of which is indicated at 49a, in the magnet 41. Proper centering of the moving coil 46 is obtained in just the same way as in the Figure 1 arrangement. In this case, and in all cases where the smaller diaphragm element is provided with a separate surround, it is desirable that the latter, the flange of the small diaphragm element to which it is secured,

and the manner in which these parts are secured together, should be such that during axial vibratory motion of the element, flexure will take place mainly at, or in the neighbourhood of, the inner edge of the surround, i. e. in the neighbourhood of the junction of the flange with the domed portion of the diaphragm. This may be attained by appropriately proportioning the flange of the smaller diaphragm element and the surround and suitably choosing the materials thereof.

It will be noted that in Figure 2, the moving coil 46 does not extend to what may be termed the inner end of the annular gap of the magnet 41. In point of fact, the arrangement is such that neither the moving coil nor its former will over reach the end of the gap during normal use of the instrument. In this case also the centre pole, indicated at 50, of the magnet 41 is of larger diameter below the gap than in the case of the Figure l arrangement, and in the space between this enlarged part of the centre pole 50 and the annular pole of the magnet is placed a layer of cotton wool, indicated at 51, which is retained loosely in position by a strap 52 of such material as adhesive tape. In this way, any possibility of dust or small particles of metal entering the gap through its inner end is obviated. It will be noted that in all cases the outer end of the gap is effectively sealed by the smaller diaphragm element against the ingress of such foreign matter.

In all loudspeakers according to the invention, it is desirable that the larger circumference of the conical diaphragm element should be mounted freely, not only as regards axial motion, but also as regards motion at any part thereof in a direction normal to that part, for it is found desirable not to restrain the tendency of the conical diaphragm element 15 to vibrate after the fashion of a bell at certain frequencies. It is for this reason that neither of the conical diaphragm elements shown in Figures 1 and 2 has a flange at its larger circumference.

The microphone instrument shown in Figure 3 of the drawings is essentially similar to the loud speaker instruments just described except that the magnet employed is somewhat, and the conical diaphragm element is considerably, smaller than in the case of the loud speakers.

As shown, a permanent magnet 61 is secured to a flanged circular base plate 62, and a domed, metal diaphragm element 63 having a movingcoil 64 secured thereto as by adhesive is mounted upon the magnet by means of screws 65 and clamping rings 66, between which latter the flange of the metal element 63 extends; the screws 65 passing through oversize holes in said flange so that proper centering of the moving coil 64 in the gap of the magnet 61 may readily be effected. A light conical diaphragm element 67 is secured by adhesive as indicated at 68 at its smaller end to the central domed portion of the metal element 63 and the larger periphery of the element 67 is turned over to provide a flange 69, said element 67 having no surround and being supported solely by its attachment to the metal element 63. The whole is enclosed by a cylindrical case 70 secured detachably at its lower end to the base plate 62 and formed at its upper end with a cir-. cular opening 71 similar in size to the larger end of the conical element 67, which opening 71 is covered by loosely woven fabric or other mate rial 72 which ofiers little or no impedance to sound waves.

One advantage of a composite diaphragm its larger periphery will according to the invention is that, by reason of the rigidity of the smaller diaphragm element and-moving coil assembly, any pressure applied to the conical diaphragm element at one point in simply result in inward movement of the smaller diaphragm element and moving-coil assembly as a whole, and not in any greater inward movement of the moving-coil at one side thereof than at the other. In other words, as it is carried by a comparatively rigid small diaphragm element, the moving-coil cannot be tilted in the gap by pressure applied unevenly to the conical diaphragm element as it could be if it were attached directly to a conical diaphragm element as is usual with known large diaphragm type sound reprodiccrs or loud speakers. This property assists greatly in rend :ing possible the use of a magnet, the annular gap of which is or" small radial width. It is found, for example, that once the small diaphragm element with the moving-coil thereon has been mounted correctly upon the magnet, and provided that the coil itself is clear of the pole pieces of the magnet, neither the actual attachment of the large conical diaphragm nor any forces vhich may later be applied to the latter can ailect the proper location of the coil in the gap, and that a clearance of even two thousandths of an inch between the coil and the pole piece is quite sufficient.

It will be observed that the nearer to the centre of the small diaphragm element the large diaphragm element is attached thereto, the more effectively will any tendency of tilting the moving coil as a result of uneven pressure upon the large diaphragm element be avoided.

Although in the case or" a microphone no surround is actually necessary for the conical diaphragm, any known means for supporting the surrounds of the two diaphragm elements may be employed; for example, it may, in the case oi some loud speakers, be desirable to employ an arrangement where the support 14 is attached to the magnet, independently of means which permit of proper centering of the moving-coil and whereby the surround of the smaller diaphragm element is clamped in position. This is very important where loud. speakers have large and heavy magnets, although the use of such large magnets is less necessary in cases where a composite diaphragm according to the invention is employed than it is where diaphragms of the usual type are employed. It will be observed that whatever happens to the large conical diaphragm element, even if it be damaged severely, it is practically impossible for the mov ng-"oil to be shifted laterally relatively to the magnet. it has, in fact, been found possible to employ successfully both in loud speakers and in microphones, magnet having a gap, the radial width of which is so small as thirty-five thousandths of an inch, thus enabling the use of a permanent magnet of so small a weight as four and a half pounds to provide an effective magnetic flux density for the moving-coil such as would, with known arrangements, have neces hated the use of a permanent magnet weighing so much as pounds.

Besides being very efiective in obtaining faithful reproduction of sound, an instrument according to the present invention possesses the advantage that the employment of a small diaphragm as above described provides a very rigid support for the moving-coil, and thus obviates all risk of the coil losing shape and becoming distorted. Further, even when the magnet is unenclosed, it is quite impossible for any foreign matter to pass from the inside of the cone between the moving-coil and the gap of the magnet, and the space between the moving-coil and the pole piece of the magnet and the inner end of the gap can easily be sealed, so that with the improved instrument there should be no risk of foreign matter entering the magnet gap causing trouble, is so often experienced with other instruments the -"noving-coils of which operate in the magnet gap with small clearance.

Thus, an instrument ccording to the invention, especially where a conical diaphragm element of paper is treated with a preservative, or where the conical element is of a material of a more permanent nature than paper, has a very long and useiul during which it will remain very 0. cut in operation without requiring any attention or maintenance whatever, and, moreover, a loud speaker according to the invention is substantially immune from a wear or damage except in the unlikely event or its being grossly overloaded or handled very roughly indeed.

What we claim is:--

l. A composite diaphragm for use in instrument of the kind referred to, comprising two elements, the one being a flanged dome having the former of the moving-coil attached thereto so that the moving-coil extends coaxially thereof at the unction of the dome with the flange, and the other being of conical formation and coaxially attached at its smaller periphery to the convex surface of the dome.

A composite diaphragm, as claimed in claim 1, wherein the one element has a surround incapable of yielding appreciably in a radial direction, and wherein the other conical element is capable of, and has a surround which permits, axial movement and also movement at any point of its larger peripheral margin in a direction other than axial.

A composite diaphragm as claimed in claim 1 wherein a flat margin of the one element is extended. and formed with a plurality of concentric corrugations to constitute a surround.

e. A composite diaphragm as claimed in claim 1 wherein a fiat margin of the one element has secured thereto a fiat annulus or" sheet material constituting a surround.

5. A composite diaphragm as claimed in claim 1, whcr a flat margin of the one element has seen d thereto a flat annular or" sheet material constituting a surround, the flat margin and the surround being such and being so secured together that during axial. motion of said element, ilexion will take place mostly at or in the neighbourhood of the inner edge of the surround.

6. A composite diaphragm for use in an instrument of the kind referred to comprising two elements, the one being a flanged dome having the former of the moving coil attached thereto so that the moving coil extends coaxially thereof at the junction of the dome with the flange, and provided with a surround incapable of yielding appreciably in a radial direction, and the other being of conical formation and coaxially attached at its smaller periphery to the convex surface of the dome and being capable of, and hav ing a surround. which permits, axial movement and also movement at any point of its larger peripheral margin in a direction other than axial, the dome having a flat, extended margin formed with a plurality of concentric corrugations to constitute a surround.

7. A composite diaphragm for use in an instrument of the kind referred to comprising two elements, the one being a flanged dome having the former of the moving coil attached thereto so that the moving coil extends coaxially thereof at the junction of the dome with the flange, and provided with a surround incapable of yielding appreciably in a radial direction, and the other being of conical formation and coaxially attached at its smaller periphery to the convex surface of the dome and being capable of, and having a surround which permits, axial movement and also movement at any point of its larger peripheral margin in a direction other than axial, the dome having a flat margin, and a flat annulus of sheet material secured to the flat margin, the flat annulus constituting a surround.

8. A composite diaphragm for use in an instrument of the kind referred to comprising two elements, the one being a flanged dome having the former of the moving coil attached thereto so that the moving coil extends coaxially thereof at the junction of the dome with the flange, and provided with a surround incapable of yielding appreciably in a radial direction, and the other being of conical formation and coaxially attached at its smaller periphery to the convex surface of the dome and being capable of, and having a sursound which permits, axial movement and also movement at any point of its larger peripheral margin in a direction other than axial, the dome having a flat margin, and a flat annulus of sheet material secured to the flat margin, the flat annulus constituting a surround, the flat margin and the surround being such and being so secured together that during axial motion of said element, flexion will take place mostly at or in the neighborhood of the inner edge of the surround.

DOUGLAS TEMPLE BENNETT. LEONARD WALTER MURKHAM. 

